Polymers are ubiquitous in current society, the most common types of polymers being used being thermosetting and thermoplastic polymers. However a third type of polymer is also used, the so-called infusible polymer (IP). These are polymers that are not crosslinked and so should theoretically be thermoplastic, but their melting and/or softening points are at a higher temperature than their decomposition temperature, so before liquefying as they are being heated, they decompose. Typically these types of polymers in commercial use have high decomposition temperatures, so their maximum use temperatures are usually quite high. Polymers of these types include, but are not limited to, polyimides, poly(p-phenylenes), and polymers composed mostly or all of repeat groups of the formula
wherein X is NH, N-Phenyl, O (oxygen) or S (sulfur), and Ar is p-phenylene, 4,4′-biphenylene or 1,4-naphthylylene.
Since these IPs cannot be formed as typical thermoplastics, the polymers are often chemically formed, and the resulting polymer, if not already a powder, is ground to a powder. This powder is then subjected to pressure and optionally heat in a mold to consolidate the powder into a shaped part. Also, optionally, the shaped part can be then sintered to further consolidate the polymer. In many ways this type of shaping process is similar to that employed in the more familiar powdered metallurgy.
Most polymers, when exposed to moisture, either as liquid water or water vapor (in the air for instance), absorb some amount of water. If the polymer is then heated rapidly to well above the boiling point of water, the absorbed water will have a considerable vapor pressure and try to escape from the polymer. If the diffusion of the water from the polymer is slow, the internal pressure of the water may cause the formation of voids within the polymer (blistering), thereby reducing or destroying the usefulness of the polymer part. For instance, if the polymer is a part of a jet engine that stands at ambient temperature in a humid climate and/or in the rain, the part may absorb a considerable amount of water. When the engine is started, sections of the engine, including where such IP parts are located, may be heated rapidly, and as a result these parts may blister. Some method of avoiding such blistering while not substantially reducing the utility of the part would be desirable.
Porous and foamed polyimides are known; see for instance U.S. Pat. Nos. 5,444,097 and 4,780,097, U.S. Published Patent Application No. 2006/0039984, and D. W. Kim et al., J. Appl. Polym. Sci. 94:1711-18 (2004). In all these references, the pores are more or less spherical (either by measurement or photograph and/or by method of preparation), and in many cases the pores are a substantial volume of the total volume of the polymer plus pores.
Japanese Patent Application 04-077533A describes a porous material characterized by being made by consolidating a matrix which may be a “resin” which includes “polyimide resin” and “unidirectional” (parallel) carbon fibers which are removed from the composite electrolytic oxidation.